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Mol. Cell. Biol., 03 1996, 1066-1073, Vol 16, No. 3
L Attisano, JL Wrana, E Montalvo and J Massague
Activin exerts its effects by simultaneously binding to two types of p
rotein serine/threonine kinase receptors, each type existing in various
isoforms. Using the ActR-IB and ActR-IIB receptor isoforms, we have
investigated the mechanism of activin receptor activation. ActR-IIB are
phosphoproteins with demonstrable affinity for each other. However, activin
addition strongly promotes an interaction between these two proteins.
Activin binds directly to ActR-IIB, and this complex associates with
ActR-IB, which does not bind ligand on its own. In the resulting complex,
ActR-IB becomes hyperphosphorylated, and this requires the kinase activity
of ActR-IIB. Mutation of conserved serines and threonines in the GS domain,
a region just upstream of the kinase domain in ActR-IB, abrogates both
phosphorylation and signal propagation, suggesting that this domain
contains phosphorylation sites required for signalling. ActR-IB activation
can be mimicked by mutation of Thr-206 to aspartic acid, which yields a
construct, ActR-IB(T206D), that signals in the absence of ligand.
Furthermore, the signalling activity of this mutant construct is
undisturbed by overexpression of a dominant negative kinase-defective
ActR-IIB construct, indicating that ActR-IB(T206D) can signal independently
of ActR-IIB. The evidence suggests that ActR-IIB acts as a primary activin
receptor and ActR-IB acts as a downstream transducer of activin signals.
Copyright © 1996, American Society for Microbiology
Activation of signalling by the activin receptor complex
Cell Biology and Genetics Program and Howard Hughes Medical Institute, New York 10021 USA.
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